Level

ABSTRACT

A level having a body with an upper surface, the upper surface having a U-shaped passage provided therein having a curved end and legs defining parallel axes, the body also having a base for supporting the body at an angle of inclination along the axes of the legs in the direction of the curved end and relative to the base; a transparent plate mounted on the upper surface in sealing relation to the passage; and fluid disposed in the passage so as to define a bubble extending through the curved end of the passage and into the legs, the bubble having ends adapted to register the horizontal or vertical disposition of the base along an axis normal to the axes of the legs when said ends of the bubble define a line of reference normal to the axes of the legs.

United States Patent [191 Bloxom Jan.'8,1974

[21] Appl. N0.: 257,176

Primary Examiner-John W. l-luckert Assistant Examiner-Milton S. Gersteinm x- Hwbn r W9 q [57] ABSTRACT A level having a body with an uppersurface, the

' upper surface having a'U-shaped passage provided therein having acurved end and legs defining parallel axes, the body also havinga basefor supporting the body at an angle of inclination along the axes of thelegs in the direction of the curved end and relative to the base; atransparent plate mounted on the upper surface in sealing relation tothe passage; and fluid disposed in the passage so as to define a bubbleextending through the curved end of the passage and into the legs, thebubble having e'nds adapted to register the horizontal or verticaldisposition of the base along an axis normal to the axes of the legswhen said ends of the bubble define a line of reference normal to theaxes of the legs;

12 Claims, 8 Drawing Figures 1 LEVEL BACKGROUND OF 'THE INVENTION Thepresent invention relates to a level and more particularly to such alevel which minimizes error due to temperature variation and imprecisionin construction, is of significantly reduced size, and is adjustable tovary its sensitivity so as to permit use ina variety of types ofsituations requiring different degrees of sensitivity.

The prior art patents such as the Castor U.S. Pat. No. 601,629; theRibalta US. Pat. No. 1,389,511; and the Schneider US Pat. No. 801,221relate primarily to levels which utilize ground glass tubes. The deviceof these patents typify prior art practices subject to deficiencieswhich the present invention has overcome, as will become more clearlyapparent.

In various areas of endeavor such as construction, grading, surveying,aviation, and the like, it is of paramount importance to be able todetermine vertical and horizontal lines and planes of reference withprecision. It has long been known to use a sealed glass tube, having aninterior surface ground in the form of an arc and gradations disposed onthe tube in corresponding spaced relation to the center point of thearc, in association with an indicator bubble within the tube forindicating horizontal or verticalattitudes. Such levels, or spiritlevels as they are sometimes called, take a variety of configurations.However, all conventional levels suffer from a variety of deficienciesirrespective of their particular configuration. These deficienciessignificantly limit their precision and therefore usefulness inoperation.

Conventional levels of substantial sensitivity are constructed in theform of a relatively long glass tube. The ratio of indicator fluid toair within the tube is normally in the neighborhood of fifty to one sothat a relatively large proportion of fluid to air is present within thetube. During use of such levels,sunlight is frequently cast directlyupon the glass tube transmitting heat to the fluid within the tube. Suchheat causes the fluid to expand within the tube and thereby compress theair therein so as to reduce the size of the indicator bubble. Thereduction in the size of the indicator bubble often results in thebubble contracting away from the spaced gradations on the tube so thatthe precise registry of level becomes difficult, if not impossible. Arelated problem resides in the fact that such sunlight can cause theglass tube itself as well as the indicator fluid to expand at one endthereof. Thus, a temperature differential between opposite ends of thetube interferes with the precision of the instrument.

The grinding of the are within the glass tubes of such conventionallevels is a quite difficult and expensive operation in order to producea level of precise capability. Furthermore, positioning of thegradations on the levels requires extreme precision in order to preventdebilitating error. However, to produce a level of even moderateaccuracy requires considerable precision in grinding of the arc andpositioning of the gradations and generally a level of relatively largedimensions. Being large, 'such levels are extremely susceptible todamage and great care must be exercised during their use. This isparticularly true at construction sites where the exposure to risk ofdamage is great.

The sensitivity of conventional levels is not adjustable due to thepermanency of the arc ground therein. As a result, a level of relativelygreat sensitivity can'be difficult to use under certain conditions wherethe work 1 object is only roughly positioned. Even a minor shift in suchan instrument produces a greatly exaggerated response in the movement ofthe indicator bubble. Thus, the bubble tends to vac'illate from one endof the tube to the other during minor adjustment making preciseadjustment quite difficult. Conversely, levels of limited sensitivitymay be convenient for such rough positioning, but of very little use inmore precise leveling. This frequently requires that two or more levelsof varying degrees of sensitivity be used in order efiiciently toperform a leveling task. This is both inconvenient and expensive.

Therefore, it has long been recognized as desirable to have an improvedlevel which minimizes distortion during use, is relatively inexpensiveto construct, is conve- SUMMARY OF THE INVENTION It is an object of thepresent invention, therefore, to

provide a level which is compact while possessing the v capability ofbeing adjusted to quite precise sensitivity.

Another object is to provide such a level which minimizes inaccuracyresulting from errors in construction as well as from environmentalfactors during use.

Another object is to provide such a level which permits reading in aflat plane.

Another object is to provide such a level which eliminates the necessityfor having gradations marked on the level which contribute to thepropensity for inaccuracy present in conventional levels.

Another object is to provide such a level which has an indicator bubblewhich is read against itself.

Another object is to provide such a level which possesses the capabilityof being adjusted so as to permit control of sensitivity from a ratherrough to a nearly infinite degree of sensitivity.

A further object is to provide such a level which can be inverted so asto permit checking for error in the level itself.

A still further object is to provide such a level which permits theacquisition of ambient'light for direction through the level so asclearly to display the indicator bubble for precise reading.

Further objects and advantages are to provide improved elements andarrangements thereof in a device for the purposes described which isdependable, economical, durable and fully effective in accomplishing itsintended purposes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of the levelof the first form of the present invention.

FIG. 2 is a vertical section, taken on line 2-2 of FIG.

FIG. 8 is a fragmentary front elevation, taken from the positionindicated by line 8-8 in FIG. 7, with the level shown in an invertedposition.

DESCRIPTION OF THE FIRST EMBODIMENT Referring more particularly to thedrawings, FIG. 1 shows the level of the first form of the presentinvention generally indicated by the numeral 10. The level is mounted ona base 11 having a substantially flat upper surface 12. The base can beof any suitable configuration depending upon the particular use forwhicl it is adapted. Preferably, it has a surface of support, not shown,which is parallel to the upper surface of the base and which is adaptedto contact the work object which is to be levelled. However, as willsubsequently be clarified, the level need not be provided with a base atall, but can be adapted for direct contact with the work object,although this eliminates at least one of its operative advantages.

As illustrated in the drawings and herein described, the level 10 ismounted on a base 11 having a rightangular light passage 13 incommunication with the exterior of the base at one of its ends andhaving a reflection opening 14 communicating with the upper surface 12thereof at the other of its ends. A mirror 15 is mounted in the passagein such a position that it collects ambient light received through thepassage and reflects such light through the reflection opening.

A pair of depressions are provided in the upper surface 12 of the base11 in positions in equally spaced relation to the reflection opening. Apair of threaded bores 21 are provided in the upper surface of the baseon opposite sides of the reflection opening so as to define a linenormal to a line defined by the depressions, as best shown in FIG. 1.

The level 10 has a substantially cylindrical body 30 secured on theupper surface 12 of the base 11. The body has a radially extendingperipheral flange 31, an upper surface 32, and an axially extendingcylindrical chamber 33 which communicates with the upper surface. Apassage 34 of somewhat smaller diameter than that of the chamber extendsaxially through the mount to communicate with the chamber so as to forma ledge 35, as best shown in FIGS. 2 and 3. A pair of threaded bores 36are provided in the flange and extends therethrough in positionscorresponding to those of the depressions 20 of the base 11. A pair ofsmooth bores 37 are also provided in the flange and extend therethroughin positions corresponding to those of the threaded bores 21 of thebase.

The body 30 is secured on the base 11 by means of a pair of adjustmentbolts 45 and a pair of mounting bolts 46. The adjustment bolts areindividually screwthreadably received in the threaded bores 36 of thebody so as to be engaged in the depressions 20 to maintain the body inadjustable spaced relation to the base, as best shown in FIG. 2. Themounting bolts are slidably received in the smooth bores 37 of the bodyand screw-threadably secured in the threaded bores 21 of the base, asbest shown in FIG. 3. One of the mounting bolts has a compression spring47 operatively interconnecting the bolt and flange. The other mountingbolt has a lever arm 48 mounted thereon and radially extending from thebody. For use, the body is mounted on the base as described at a slightangle of inclination along an axis defined by the mounting bolts in thedirection of the bolt having the lever arm. The arm is adapted to beemployed selectively to thread the bolt into or out of the threaded bore21 a short distance against compression of thespring 47 of the otherbolt. Such adjustment permits increase or decrease of the angle ofinclination of the body with respect to the base, the significance ofwhich will subsequently be made more clearly apparent. When the body ismounted on the base, as described, the passage 34 of the body is inaxial alignment with the reflection opening 14 of the base, as bestshown in FIG. 2.

An indicator unit 55 is received in the cylindrical chamber 33 of thebody 30 in resting relation on the ledge 35. The unit has a cell 56having parallel upper and lower surfaces 57 and 58, respectively. Anelongated passage 59, communicating with the upper and lower surfaces ofthe cell, is provided in the cell defining a longitudinal axis inalignment with the mounting bolts 46 of the body 30. A divider wall 60is mounted longitudinally within the passage, as best shown in FIG. 2,so as to divide the passage in half and to define a curved end oropening 61 at the -end thereof adjacent the lever arm 48. The passage isthereby made substantially U-shaped.

A pair of transparent plates 65 are individually mounted in parallelrelation on the upper and lower surfaces 57 and 58, respectively, of thecell 56. The plates are mounted with seals 66 individually disposedbetween both plates andthe cell so as to retain the plates influid-tight sealing relation to the divider wall 60 and cell. Apredetermined quantity of a suitable indicator fluid 67, such as ether,is provided in the passage 59 so as to define a bubble 68 therein. Thebubble is relatively long and retum-bent or substantially U- shaped,having parallel legs 69 with ends 70, when correctly positioned.Adjustment of the angle of inclination of the body 30 with respect tothe base 11 serves more importantly to adjust the angle of inclinationof the indicator unit with respect to the base. With a slight angle'ofinclination in the direction of the lever arm, the bubble is disposed inits desired U-shaped configuration extending through the opening 61 withthe legs thereof on opposite sides of the divider wall. I

A top plate having a passage 76 extending axially therethrough ismounted on the upper surface 32 of the body 30 by bolts 77. The topplate is mounted on the upper surface so that it is partially receivedwithin the cylindrical chamber 33 so as to engage the indicator unit 55to maintain it in place within the chamber. A transparent refractionplate 78 is received in the passage of the top plate in engagement withthe upper transparent plate of the indicator unit, as best shown in FIG.2. The refraction plate has an upwardly displayed V-shaped groove 79.The refraction plate is positioned so that the vertex of the groove ispositioned in precise parallel alignment with the divider wall 60 of theindicator unit.

DESCRIPTION OF THE SECOND EMBODIMENT It will be seen that the presentinvention can take a variety of forms, depending upon the particular jobrequirements. The indicator unit 55 having the U-shaped bubble 68displayed therein, can be mounted for use in any one of a wide varietyof ways. That described in regard to the first embodiment of theinvention is merely representative of one particular form shown forillustrative convenience. Furthermore, all that is essential in any formof the invention is the indicator unit itself utilizing areturn-bentbubble. Thus, the construction of the passage 59 with the divider wall60 mounted therein, and the equivalent structure of the second form ofthe invention to be described, could be replaced by a return-bent grooveor similar structure in order to make possiblea retum-bent bubble. It isnot necessarily required that provision be made for adjusting the angleof inclination of the indicator unit. Thus, an indicator unit can beconstructed with a built-in angle of inclination for use inpredetermined types of levelling operations without requiring use of thebase 11.

The second form of level 100, shown in FIGS. 6, 7

and 8, is intended to be of a somewhat more sophisti-.

cated form for use on, for example, surveying instruments. Such aninstrument is indicated by the numeral 101 and has spaced parallel arms102 providing longitudinally aligned bearings 103 mounted therein. Abase 104 is rotationally mountedin the bearings and has a substantiallyflat platform 105 integrally provided thereon and disposed between thearms.

A lock mechanism is mounted at one end of the base in releasableengagement with the adjacent arm 102, as best shown in FIG. 7. A pair ofslots 111 are provided in the arm 180 apart about the periphery of thebearing 103 and defining a true vertical line when the instrument 101 isproperly positioned. The lock mechanism has a spring tension sleeve 112which releasably engages the slots of the arm. Utilizing the lockmechanism, the platform 105 can be locked in two positions precisely 180apart with respect to the axis of rotation. The base has a right-angularlight passage 113 extending from the opposite end thereof to its center,as best shown in FIG. 7. The passage communicates with the exterior ofthe base for the collection of ambient light. The passage similarlycommunicates with the platform by way of a reflection opening 114positioned concentrically of the platform. A mirror 115 is mountedwithin the passage so as to reflect ambient light received through thepassage upwardly through the reflection opening.

As in the first form of the invention, a pair of depressions areprovided in spaced relation on opposite sides of the reflection opening114 and define a line parallel to the axis of rotation of thecylindrical base 104. A pair of threaded bores 121 are also provided inthe platform in spaced relation on opposite sides of the opening anddefining a line normal to that of the depressions.

The level 100 has a body having a peripheral flange 131, an uppersurface 132, and a centrally positioned cylindrical chamber 133 whichcommunicates with the reflection opening 114 through a smaller diameterpassage 134. The smaller diameter of the passage with respect to that ofthe chamber defines a ledge 135. A pair of threaded bores 136 inpositions corresponding to the depressions 120 and a pair of smoothbores 137 in positions corresponding to the threaded bores 121 areprovided in the flange of the body. A pair of adjustment bolts areindividually screwbolts and the flange and a lever arm 148 is providedon the other mounting bolt.

An indicator unit is received within the chamber 133 in resting relationagainst the ledge 135. As in the first form of the invention, the unitincludes a cell 156 having upper and lower surfaces 157 and 158,respectively. The cell has an elongated passage 159 extending in axialalignment with a line defined by the mounting bolts. A divider wall 160is mounted longitudinally within the passage so as to divide the passagein half and to define openings 161 at the opposite ends thereof. Thus,the divider wall defines a circular path about the wall within thepassage.

The cell 156 has transparent plates 165 individually mounted on theupper and lower surfaces 157 and 158, respectively, in sealing relationto the passage 159 by means of seals 166 disposed therebetween. Asuitable indicator fluid 167, such as ether, is provided in the passageso as to define a bubble 168 having legs 169 and ends 170. Ambient'lightis reflected through the indicator unit 155 by the. mirror 155 of thebase 104.

A top plate having a passage 176 therethrough is mounted on the uppersurface 157 of the cell 156 by means of bolts 177 to retain theindicator unit 155 in position. A transparent refraction plate 178 isreceived in the passage in engagement with the transparent plate unit155 from the mirror 155. A mirror 187 is mounted within the housing soas to reflect light received through the refraction plate outwardly fromthe housing through the lens.

OPERATION I The operation of the described embodiments of the subjectinvention is believed to be clearly apparent and is briefly summarizedat this point. Both forms of the threadably received in the threadedbores 136 so as to invention 10 and 100 operate in substantially thesame manner. In each instance, the level is positioned against a workobject to be levelled. In the caseof level 10, this positioning isaccomplished in the conventional manner by direct contact of the base 11with the work object. In the case of level 100, the instrument 101 has astructure, not shown, such as a tripod for contacting the work object.In both cases, the level is oriented so that the passages 34 and 134 areright-angularly related to the line along which levelling is to beaccomplished.

Depending upon what particular levelling operation is to be performed,the lever arms 48 and 148 of both forms of the invention are adjusted soas to achieve the desired angle of inclination of the indicator units 55and155, respectively. In the first form of the invention, the angle ofinclination disposes the bubble 68 so that it extends through theopening 61 in a U-shaped configuration on opposite sides of the dividerwall 60. In the second form of the invention the bubble 168 extendssimilarly through one of the openings 161 when the platform is in anupwardly facing position and through the opposite opening when theplatform is in an inverted position. The magnitude of the angle ofinclination to which the unit is adjusted is inversely proportional tothe sensitivity produced in the unit. Thus, for initial levelling or forthe levelling of work objects in circumstances in which extreme accuracyis not required nor perhaps desired, the angle of inclination should beadjusted using the lever arm, so that it is of a relatively greatmagnitude. Gradations, not shown, can be provided on the flange belowthe lever arm so that the precise angle of inclination desired can beselected by movement of the lever arm above the selected gradation.

In both forms of the invention, reading of the indicator units 55 and155, respectively, is accomplished in the same manner. After selectionof the sensitivity desired using the lever arms 48 and 148,respectively, and positioning of the level so that the passages 59 and159 are disposed normal to the line along which leveling is to beaccomplished, the level is oriented so as to position the ends 70 and170 of the bubbles 68 and 168 respectively in precise alignment. .Therefraction plates 78 and 178 of each form of the invention reflect thelight of the remote halves of the ends of the bubble so as to disposethem in side-by-side relation, as indicated diagrammatically in FIG. 5.The image presented then is as shown in FIG. 1 so'that precise alignmentof the ends of the bubbles can be accomplished relatively easily. Ofcourse, when the ends are in precise alignment, level has beendetermined within the range of sensitivity controlled by the adjustmentof the lever arms 48 and 148.

Where more precise adjustment is desired the lever arms 48 and 148,respectively, can be adjusted to decrease the angle of inclination ofthe indicator units 55 and 155 thereby increasing the sensitivity. Thepotential for adjusting the levels 10 and 100 to increase sensitivity isnearly infinite. As the angle of inclination approaches preciselyhorizontal, the tendency of the bubble to float within the unit towardthe raised end' thereof becomes increasingly less pronounced. Of course,when true horizontal is reached, the bubble will no longer float to araised end of the unit and therefore will break into segments within thepassages 59 and 159 so as immediately to indicate that true horizontalhas been reached by the indicator units as distinct from the base 11 and104. The goal in adjusting the indicator units to the most sensitiveadjustment is to approach as nearly as possible without reaching precisehorizontal so that the bubble is still intact at the raised end of thepassage.

It will be noted that through the use of the light passages 13 and 113and the mirrors l and 115, ambient light in the area is collected andreflected through the indicator units 55 and 155 so that the imagepresented to the operator is sufficiently illuminated to permit precisereading of the level. It has been found that use of only one opening 61at an end of the divider wall 60 in the first form of the inventionpermits the transparent plates 65 thereof to be constructed of glassother than that which has been optically ground without danger ofsignificant error.

The level of the second form of the invention 100 differs from the firstform in that provision is made for inversion of the indicator unit so asto permit'checking of the unit 155 against itself for error. With thelevel disposed in the relationship shown in FIGS. 6 and 7, a firstreading is taken by viewing through the lens 186. The lens is adjustableso as to permit precise focusing in order to obtain a precise reading.When a reading has been obtained by this means, as above described, theindicator unit can be inverted precisely to take a reading with thebubble against the opposite transparent plate 165. Inversion of theindicator unit is accomplished by grasping the sleeve 112 of the lockingmechanism 110 to draw the sleeve out of engagement with the slots 111and to rotate the cylindrical base 104 in the bearings 103 until thesleeves again engage the slots 111 and the platform 105 has beeninverted 180, as shown in FIG. 8. With the inversion, the bubble 168floats to the opposite transparent plate and to the opening 161 oppositeto that through which the bubble extended when reading in the priorposition. This is due to the reversal of the angle of inclination as aresult of rotation. The lens is subsequently adjusted to accommodate thedifference in position in the bubble so as again to permit precisefocusing. It will be noted that adjustment of the lever arm 168 can beaccomplished in either position so as to permit adjustment of the angleof inclination of the indicator unit and thereby the sensitivity of thelevel.

Although the invention has been herein shown and described in what areconceived to be the most practical and. preferred embodiments, it isrecognized that departures may be made therefrom within the scope of theinvention, which is not to be limited to theillustrative detailsdisclosed.

Having described my invention, what I claim as new and desire to secureby Letters Patent is:

l. A level for determining a pre-selected attitude comprising a bodydefining a plane of reference, the body having an upper surface; andindicator means mounted on the upper surface and defining a first axisadjacent to but canted relative to the plane of reference, and adaptedto indicate when the plane of reference is disposed in the pre-selectedattitude along a second axis normal to the first axis of the indicatormeans, said indicator means including a return-bent passage in the uppersurface of the body, said passage having fluid disposed therein so as todefine an indicator bubble having ends adapted to be disposed insideby-side relation in the retum-bent passage when the plane ofreference is oriented in the pre-selected attitude.

2. The level of claim 1 in which the ends of the bubble defined by thefluid have predetermined corresponding portions and a refraction plateis mounted in covering relation to the passage so as to represent thecorresponding portions of the ends of the bubble in juxtaposition whenviewed through the plate to allow precise reading of the level.

3. A level for registering horizontal along a first axis comprising abody having'upper and lower surfaces, and a longitudinal passage formedin the body defining a second axis right-angularly related to the firstaxis; a

wall mounted within a longitudinally of the passage to the wall, thebubble having ends which register the horizontal disposition of saidmeans along the first axis when said ends define a line parallel to thefirst axis.

4. The level of claim 4 in which the passage and wall extend through thebody and a second transparent plate is mounted on the lower surface ofthe body in sealing relation to the passage and wall adapted to permitambient light to pass through the passage to assist in the observationof the disposition of the ends of the bubble.

5. The level of claim 3 in which the means includes a base having meansfor adjustably supporting the body for varying the angle of inclinationof the body with respect to the base in order to adjust the tendency ofthe bubble to float to the opening end of the passage and thereby thesensitivity of the level.

6. The level of claim 5 in which the means for adjustably supporting thebody includes a bolt which interconnects the body and the base inalignment with the second axis for increasing or decreasing the angle ofinclination so as to adjust the sensitivity of the level from roughsensitivity to a precise sensitivity.

7. The level of claim 6 in which the wall provides a second opening atone end thereof opposite that defining the first opening in order tofacilitate movement of the bubble in the passage.

8. A level for use on levelling instruments and the like for positioningin vertically standing relation so as to permit the determination ofhorizontal along a first axis, the level comprising a base mounted onthe instrument so as to be positionable in an upwardly facing positionand an inverted position; a body having opposite sides and a passageelongated along a second axis normal to the first axis; a wall mountedin the passage'in alignment with the second axis so as to divide thepassage and define openings at the opposite ends of the wall; a pair ofparallel'transparent plates individually mounted on the opposite sidesof the body in sealing relation to the passage and wall; fluid receivedin the passage about the wall so as to define an indicator bubble havingopposite ends adapted to register horizontal when the ends define a lineparallel to the first axis; and the body being mounted on the base andinclined along the second axis so as buoyantly to motivate the bubble inthe direction of the inclination and said body being positionable withthe base in the upwardly facing and inverted positions so as alternatelyto motivate the bubble to opposite ends of the passage for the alternateregistry of horizontal to permit a check for error within the levelitself.

9. The level of claim 8 in which an adjustment bolt interconnects thebody and the base for adjustment of the inclination of the body withrespect to the base to permit varying the degree of sensitivity of thelevel.

10. A level for determining a pre-selected attitude comprising a bodyhaving a plane of reference; indicator means mounted on the body, saidindicator means having a return-bent passage disposed in a planeadjacent to but canted relative to the plane of reference;

and fluid disposed in the passage so as to define a re-.

tum-bent bubble having ends which register the attitude of the plane ofreference with respect to the horizontal by their relationship withrespect to each other. 11. A level comprising a base having apredetermined erect plane of reference providing therein a predeterminedaxis of reference with respect to which the horizontal is to becompared; a substantially U-shaped' transparent tube having adjacentlegs interconnected by a return-bent portion mounted in the base in aplane which passes through the axis of reference, with the legs disposedtransversely of the axis of reference and inclined toward thereturn-bent portion when the plane of reference is vertical; and a fluiddisposed in the tube defining a return-bent bubble having opposite endsdisposed in the opposite legs for visual comparison and which registerwhen the axis of reference is horizontal.

12. In a level, an indicator comprising a substantially U-shapedtransparent tube having legs in adjacent proximity interconnected by areturn-bent portion, and a fluid partially filling the tube so as todefine a retumbent bubble having opposite ends disposed in the legs forvisual comparison of the positional relationship of said opposite endsof the bubble.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 a 783a 525 I Dated January 8 a 1974' Inventor(s) C. W. Bloxom It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

In the Specification:

Col. 3 line 11, delete "whicl" and insert -which-- Col; 6, line 21,after "mirror", and before "of" delete "1.55" and insert --].].5

Col. 6, line 35, after "mirror" and before the period del ete "-155" andinsert -ll5- In the Claims:

Col. 8, line 57, after "within" and before "longitudinally" delete "a"and insert ---andcol. 9, line 1, delete "'4" and insert ---3---.

Signed and I sealed this 16th day of April IJ'ZL (SEAL) Attest: v

EDWARD I LFLETGHERJR. C IQARSI-LALLDANBT Attesting Officer Commissionerof Patents FORM 9 (10'69) I V '1 USCOMM-DC 60376-P69 a u.s. sovznnmzmrnim'mc ornci: 1 was muss-an.

1. A level for determining a pre-selected attitude comprising a bodydefining a plane of reference, the body having an upper surface; andindicator means mounted on the upper surface and defining a first axisadjacent to but canted relative to the plane of reference, and adaptedto indicate when the plane of reference is disposed in the pre-selectedattitude along a second axis normal to the first axis of the indicatormeans, said indicator means including a return-bent passage in the uppersurface of the body, said passage having fluid disposed therein so as todefine an indicator bubble having ends adapted to be disposed inside-by-side relation in the return-bent passage when the plane ofreference is oriented in the pre-selected attitude.
 2. The level ofclaim 1 in which the ends of the bubble defined by the fluid havepredetermined corresponding portions and a refraction plate is mountedin covering relation to the passage so as to represent the correspondingportions of the ends of the bubble in juxtaposition when viewed throughthe plate to allow precise reading of the level.
 3. A level forregistering horizontal along a first axis comprising a body having upperand lower surfaces, and a longiTudinal passage formed in the bodydefining a second axis right-angularly related to the first axis; a wallmounted within a longitudinally of the passage to divide the passage inhalf and define an opening at one end thereof; a transparent platemounted on the upper surface in sealing relation to the passage andwall; means supporting the body at an angle of inclination in thedirection of the opening along the second axis; and fluid disposed inthe passage so as to define a bubble extending through the opening andon opposite sides of the wall, the bubble having ends which register thehorizontal disposition of said means along the first axis when said endsdefine a line parallel to the first axis.
 4. The level of claim 4 inwhich the passage and wall extend through the body and a secondtransparent plate is mounted on the lower surface of the body in sealingrelation to the passage and wall adapted to permit ambient light to passthrough the passage to assist in the observation of the disposition ofthe ends of the bubble.
 5. The level of claim 3 in which the meansincludes a base having means for adjustably supporting the body forvarying the angle of inclination of the body with respect to the base inorder to adjust the tendency of the bubble to float to the opening endof the passage and thereby the sensitivity of the level.
 6. The level ofclaim 5 in which the means for adjustably supporting the body includes abolt which interconnects the body and the base in alignment with thesecond axis for increasing or decreasing the angle of inclination so asto adjust the sensitivity of the level from rough sensitivity to aprecise sensitivity.
 7. The level of claim 6 in which the wall providesa second opening at one end thereof opposite that defining the firstopening in order to facilitate movement of the bubble in the passage. 8.A level for use on levelling instruments and the like for positioning invertically standing relation so as to permit the determination ofhorizontal along a first axis, the level comprising a base mounted onthe instrument so as to be positionable in an upwardly facing positionand an inverted position; a body having opposite sides and a passageelongated along a second axis normal to the first axis; a wall mountedin the passage in alignment with the second axis so as to divide thepassage and define openings at the opposite ends of the wall; a pair ofparallel transparent plates individually mounted on the opposite sidesof the body in sealing relation to the passage and wall; fluid receivedin the passage about the wall so as to define an indicator bubble havingopposite ends adapted to register horizontal when the ends define a lineparallel to the first axis; and the body being mounted on the base andinclined along the second axis so as buoyantly to motivate the bubble inthe direction of the inclination and said body being positionable withthe base in the upwardly facing and inverted positions so as alternatelyto motivate the bubble to opposite ends of the passage for the alternateregistry of horizontal to permit a check for error within the levelitself.
 9. The level of claim 8 in which an adjustment boltinterconnects the body and the base for adjustment of the inclination ofthe body with respect to the base to permit varying the degree ofsensitivity of the level.
 10. A level for determining a pre-selectedattitude comprising a body having a plane of reference; indicator meansmounted on the body, said indicator means having a return-bent passagedisposed in a plane adjacent to but canted relative to the plane ofreference; and fluid disposed in the passage so as to define areturn-bent bubble having ends which register the attitude of the planeof reference with respect to the horizontal by their relationship withrespect to each other.
 11. A level comprising a base having apredetermined erect plane of reference providing therein a predeterminedaxis of reference with respect to which the horizontal is to Becompared; a substantially U-shaped transparent tube having adjacent legsinterconnected by a return-bent portion mounted in the base in a planewhich passes through the axis of reference, with the legs disposedtransversely of the axis of reference and inclined toward thereturn-bent portion when the plane of reference is vertical; and a fluiddisposed in the tube defining a return-bent bubble having opposite endsdisposed in the opposite legs for visual comparison and which registerwhen the axis of reference is horizontal.
 12. In a level, an indicatorcomprising a substantially U-shaped transparent tube having legs inadjacent proximity interconnected by a return-bent portion, and a fluidpartially filling the tube so as to define a return-bent bubble havingopposite ends disposed in the legs for visual comparison of thepositional relationship of said opposite ends of the bubble.